1. Field of the Invention
During the manufacture of many thermoplastic films, especially films of polyethylene terephthalate, a molten film web is cooled by extrusion onto a quench roll casting drum. Uniform pinning of the web to the quench roll is facilitated by the removal of air proximate the line of contact between the web and the quench roll by vacuum forces to eliminate such air from being trapped under the web during the lay-down so as to mitigate irregularities in the uniformity of cast sheeting, especially prominent at high speed operation. This invention is directed toward improvements in eliminating air pockets and reducing irregularities caused by air turbulence resulting from the vacuum forces applied to the line of contact between the quench roll and the web.
2. Description of Prior Art
In the production of film from certain thermoplastic polymers in which such film is formed at temperatures above or near the polymer melting point, it is important to quickly cool the freshly formed film to a temperature below the second order transition temperature, i.e., that temperature at which, as temperature decreases, the polymer goes from a flexible state to a more rigid, glassy state, in order to prevent crystallization which would interfere with subsequent orientation of the film. A number of devices have been employed to create more intimate contact between the web and the quench roll, the most prominent being, generally, forced air pinning, electrostatic pinning, and vacuum pinning. Forced air pinning, where the molten web is urged against the quench roll by means of air pressure exerted against the upper surface of the cast web, is of limited use during high speed operation due to film distortions created by air being trapped under the molten web as the web is laid onto the quench roll. Electrostatic pinning, where the cast web is held to the quench roll by means of electrostatic forces, has also been limited to slower operation as air bubbles are trapped under the web at high speeds.
In the recent past, the removal of the air proximate the contact line between the web and the quench roll has been attempted with partial success by vacuum forces which achieve varying degrees of uniformity in the pinning process. A major weakness in the use of these vacuum forces stems from air turbulence created by the vacuum, causing the web to vibrate as it flows onto the quench roll, such vibration resulting in a lack of uniformity in the casting and cooling process and hence, the web product. Prior art has exhibited some reduction of air turbulence by utilizing successive vacuum zones with porthole like interconnections or with individual vacuum forces, however, significant air turbulence at the line of contact between the web and the quench roll and consequent pinning and quenching irregularities remain. Further, in prior art devices, ambient air movement drawn past the edges of the extruded web, hereinafter "edge leak air", is essentially straight back, parallel with the movement of the casting drum. This leak air movement gives rise to significant air turbulence proximate the web edge bead and has been shown to bend the edge of the molten web back as the web is flowing from the die lip to the quench roll giving the web edge a scalloped appearance. This scalloping of the edge of the web before it is laid onto the quench roll is referred to herein as bead wobble. In prior art devices, edge leak air necessarily enters the vacuum zone proximate the molten web through the opening between the edge of the molten web and the vacuum device, thus, creating air turbulence within the device proximate the extruded web. Prior art devices, while mitigating air turbulence and, therefore, vibration of the extruded web, still leave much to be desired at high film manufacturing speeds. No prior suggestion of devices similar to this invention is known wherein there are two vacuum zones interracting only through a close fitting baffle, or skimmer;--one zone, hereinafter the primary zone, being proximate the line of contact of the web onto the casting drum, the other zone, hereinafter the secondary zone, being distal such line of contact relative to the primary zone, where the vacuum forces acting on the secondary zone do not create air turbulence within the primary zone due to the skimmer. Further, no prior suggestion is known to further mitigate bead wobble by extending the secondary zone to encompass both edges of the web as it is laid onto the drum by use of edge seals in conjunction with a quiescent vacuum zone such that the edge leak air drawn into the edge seal is substantially perpendicular to the movement of the casting drum.